In conventional motor vehicles, a 14V vehicle electrical system is supplied with electrical energy via a 14V generator. The generator is generally a three-phase or multiphase electric machine, which is driven by the internal combustion engine of the motor vehicle and generates polyphase current which is rectified by a rectifier.
Since the excitation winding of the generator has a very high inductance, during an abrupt load drop, which is referred to as a “load dump,” initially an unreduced electric current continues to be fed into the vehicle electrical system and a high voltage continues to be generated. Depending on the capacitance present in the vehicle electrical system, the voltage value exceeds the maximum voltage limit of the vehicle electrical system within a few milliseconds. The generator current decays at the time constant of the excitation field, resulting in a maximum load dump time of several 100 ms.
A rectifier may be formed of Zener diodes, which during normal operation act as current valves and cause a rectification, but in the special case of a load dump may also limit the occurring electric overvoltages by conductive the generator current to ground via the Zener breakdown, instead of feeding it into the vehicle electrical system. This is referred to as passive rectification, and this type of voltage limitation is referred to as voltage clamping.
In the case of an active rectification, each diode is replaced with a power MOSFET including an intrinsic body diode, which is antiparallel to the channel of the power MOSFET between the drain and the source and functions without control of the gate of the MOSFET in a manner identical to a diode rectifier. Through suitable, rapid voltage forcing with the aid of a gate driver, the MOSFET may always be switched on whenever the phase current is to flow through it, i.e., the intrinsic diode of the MOSFET is short-circuited by the channel of the MOSFET. Compared to the passive rectifier, a considerably reduced forward voltage arises at the source-drain channel in this way, and accordingly the efficiency and the output power of the generator at low rotational speeds are increased. A rapid control of the MOSFET is needed to actually switch at the zero crossing so as not to generate additional ripple of the rectified output voltage. Both a fast evaluation of the phase voltage and a sufficiently high gate driver current are needed for this purpose, i.e., a preferably low-resistance control of the gate.
In the case of a load dump, an evaluation circuit detects an electric overvoltage at the positive pole of the active rectifier and electrically short-circuits the connected phase to the reference potential (ground) or to the positive pole of the active rectifier. In a multiphase system, the phase short circuit is brought about, either autonomously for each phase or controlled via a synchronization line, on all further phases so that the generator no longer feeds any electric current into the vehicle electrical system.
If voltage gradients at the MOSFET gate are too steep during the deactivation/activation of the phase short circuit, the large gradients of the generator current, in conjunction with the cable inductance of the connecting line between the capacitance in the vehicle electrical system and the generator, cause high voltage peaks, in absolute terms, in the case of an active rectifier. These may damage a control unit of the generator. The switching process for deactivation/activation of the phase short circuit is therefore advantageously carried out slowly, i.e., with a long switching time.
During normal rectifier operation, however, switching processes must be carried out quickly, i.e., with a shorter switching time, in particular in cases where operation takes place at high rotational speeds and high generator currents.
Switching denotes the transition between the “conductive” and “non-conductive” states. A “long switching time” or “slow switching” in the context described here denotes a slow transition between the “conductive” and “non-conductive” states; in contrast, a “short switching time” or “fast switching” denotes a fast transition.
Slow switching may be achieved during the phase short circuit by high-resistance, fast switching, and during the active rectification by a low-resistance control of the MOSFET gate.
In order to meet these two requirements—high-resistance control with deactivation/activation during the phase short circuit and low-resistance control during the active rectifier operation—a switch takes place between two driver mechanisms. During normal operation, a voltage is provided with low resistance, while during phase short circuit operation a precisely set current source is connected to the output.
The crucial aspect is when a change between high-resistance and low-resistance operation is carried out. Switching at the wrong point in time may result in undesirably fast switching, and thus in voltage peaks, which may destroy the electrical components of the control unit.
A need therefore exists for a method for determining the point in time for a switch in the control.